Liquid Crystal displays (LCD) of the OCB (optically compensated bend) are based on a birefringence effect and comprise a liquid crystal (LC) layer with a bend structure. The bend cell, also known as pi-cell, was first proposed by P. Bos et al., SID 83 Digest, 30 (1983) for an electrically controllable half-wave plate, whereas the OCB mode for displays was described by Y. Yamaguchi, T. Miyashita and T. Uchida, SID 93 Digest, 277 (1993), followed by papers of T. Miyashita et al. in, inter alia, Proc. Eurodisplay, 149 (1993), J. Appl. Phys. 34, L177 (1995), SID 95 Digest, 797 (1995), and C.-L. Kuo et al., SID 94 Digest, 927 (1994). An OCB cell typically comprises an LC cell with bend alignment and an LC medium with positive dielectric anisotropy Δε. Furthermore, OCB displays as reported in the above papers comprise one or more birefringent optical retardation films to eliminate light leakage by the bend cell in the black state. OCB displays bear several advantages like for example a wider viewing angle and shorter switching times than conventional LC displays based on twisted nematic (TN) cells. They are therefore a candidate for video-rate applications such as LCD televisions. Television applications require large display areas and hence wide viewing angles.
Usually an OCB mode display requires optical compensation films to provide both on-axis contrast and wide viewing angles.
Compensators comprising polymerised LC materials have been suggested in prior art. For example, P. Van de Witte et al. SID 97, pp. 687-690 and U.S. Pat. No. 5,619,352 disclose compensators with A plate, O plate and C plate symmetry and combinations thereof for viewing angle compensation of TN-LCDs, wherein the compensation films may comprise polymerised calamitic LC material with splayed or planar orientation. WO 01/20393 discloses combinations of splayed, planar and highly twisted polymerised LC films with optical A plate, O plate and negative C plate symmetry for viewing angle compensation of LCDs. However, it has turned out that these compensators, when used in LCDs of the OCB type, still do not show satisfying performance and in particular give insufficient viewing angle compensation.
K. Vermeirsch et al. IDRC 2000, poster 56A discloses a compensated OCB-LCD comprising a combination of anti-parallel aligned, splayed and planar compensators orientated perpendicular to the OCB cell, however, this combination only yielded a narrow viewing angle.
US 2002/0063828 discloses optical compensation films made from discotic LC materials for use in LCDs. However, the dispersion mismatch between the discotic compensator and the calamitic LC in the switchable display cell produces colouration.
One aim of the present invention is to provide a compensator for an LCD of the OCB or pi-cell mode that improves the optical performance of the LCD, in particular the contrast at wide viewing angles, is easy to manufacture, and allows economic fabrication even at large scales.
Another aim of this invention is to provide an advantageous use of the compensator according to this invention.
A further aim of this invention relates to OCB and pi-cell LCDs comprising an inventive compensator which show advantageous properties such as good contrast, reduced colour shift and wide viewing angles.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The above aims can be achieved by providing compensators and LCDs according to the present invention.